Detecting seasonal differences in high-frequency site response using κ0

Near-surface geologic site conditions significantly affect seismic waves by amplifying certain frequency ranges and attenuating others. For seismic hazard analysis, site conditions are assumed to be constant over time. Contrary to this assumption, temporal variations in near-surface velocities have been observed in recent years. This study shows for the first time that ∆κ0, the site component of the spec- tral decay parameter κ derived between a surface and a borehole sensor, can vary seasonally. ∆κ0 is an integrative parameter of local site attenuation and amplification at high frequencies. Using data from the Kiban Kyoshin Strong Motion Network (KiK-net) in Japan, we analyze recordings of seismic events between 2004–2020 and correlate temporal ∆κ0 variations with environmental factors such as temperature, precipi- tation, snow depth, soil moisture, and terrestrial water storage. We can identify strong seasonal variations at 13 sites in northeastern Hokkaido and on Honshu, with ∆κ0 being generally larger in winter than in summer. Our results indicate that the high-frequency site response can be influenced by environmental conditions and should not be assumed to be constant